FLIP is a caspase-8 homologue that blocks apoptosis by death receptors. Cellular FLIP (c-FLIP) occurs in two isoforms, full-length long FLIP (c-FLIPL) and a truncated short FLIP (c-FLIPS). The reason for these two isoforms in not known. We have recently defined a new role for FLIP in T cell growth. In the absence of Fas ligation in activated non-dying T cells, c-FLIPL heterodimerizes with and activates caspase-8. Caspase-8 activity is in fact required for proliferation of primary T cells, c-FLIPL also binds RIP which activates NF-kB. Both c-FLIPL and RIP are cleaved by caspase-8. Cleaved c-FLIPL (p43FLIP) more effectively binds RIP and this may partly explain the caspase-8 requirement for T cell activation. If levels of c-FLIPL are elevated, then excessive caspase-8 activation occurs and prematurely cleaves RIP, thus turning off the NF-kB signal. This results in increased cell death, especially of CD8+ T cells, and a bias toward Th2 cytokines in the CD4+ subset. By contrast, short FLIP (c-FLIPS) is not able to activate caspase-8 as it lacks the critical loop that opens the caspase-8 active site but it still efficiently recruits RIP. We thus predict that elevated expression of c-FLIPS will lead to persistent activation of NF-kB, with a phenotype that is rather opposite to that of the c-FLIPL-Tg mice, and will promote survival of memory T cells. This model is studies in four Specific Aims. Aim 1. How does c-FLIPL promote a Th2 response in CD4+ T cells? Naive c-FLIPL-Tg CD4+ T cells manifest increased GATA-3 and decreased NF-kB activity. Each of these will be reversed using transgenic and retroviral approaches. Cytokine expression on a per cell basis will be determined by crossing the c-FLIPL- Tg mice with eGFP/IL-4 and eYFP/IFN-gamma reporter mice. Aim 2. What is the significance of caspase-dependent cleavage of c-FLIPL and RIP on NF-kB activity, proliferation, and cytokine phenotype of CD4+ T cells? Both c-FLIPL and RIP will be mutated at their critical aspartate cleavage sites to non-cleavable forms and expressed transgenically and retrovirally. Aim 3. Does c-FLIPS promote sustained signaling of NF-kB, leading to increased memory T cell survival? NF-kB and caspase activities will be determined in c-FLIPS-Tg CD4+ T cells. Generation of effector and memory T cells will be compared between wild type and c-FLIPS-Tg CD4+ T cells. Aim 4. How do c-FLIPL and c-FLIPS affect EAE and asthma? The Th2 bias may improve EAE and worsen asthma.